1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and an ink-jet printing method using this apparatus, and more specifically to handling a conveyance error generated when a rear end of a printing sheet exits from a nipped portion of a conveying roller.
2. Description of the Related Art
Recently, personal computers, word processors and facsimile apparatuses have become widely used in offices and homes, whereby various kinds of printing apparatuses have been provided as information outputting equipments for these apparatuses. Of them, printers of an ink-jet type are relatively easily accommodated to color printings using a plurality of kinds of ink. Also, the printers of this type have various advantages, such as low in noise during the operation and capable of forming high grade images on various kinds and types of printing sheets or small in size. In view thereof, the printers of this type are suitable for the business use in offices or the personal use in homes. Among the printing apparatuses of the ink-jet type, those of a serial type wherein scanning of a printing head is performed to a printing sheet to print an image thereon have been widely used for a high grade image being printable at a low cost.
While the serial printing apparatus is relatively low in cost on one hand, high printing performances are desired on the other hand. Particularly, one of the desired printing performances is typically the printing of high quality image. One factor for obtaining the high quality image is a positional accuracy of landed ink dots. In general, while a factor for determining the positional accuracy of the landed ink dot may be the ejection stability of the printing head such as a non-ejection or a displacement of the ejection direction, in the recent high quality image printing, the conveying accuracy of the printing apparatus itself for conveying the printing sheet becomes important.
Conventionally, the printing apparatuses such as printers, copiers or facsimile apparatuses, use a conveying roller and pinch roller for generating a conveying force by pressing the printing sheet onto the conveying roller and nipping the same between both the rollers, as means for conveying the printing sheet. Also, a mechanism may be provided for generating a bias for urging the pinch roller as described above. This conveying mechanism conveys the printing sheet fed from a paper feeding section to a printing area by the printing head, wherein a set of the mechanism is generally provided at each of positions before and after the printing area, respectively. Thereby, it is possible to convey the printing sheet along the printing area at a high accuracy and impart the predetermined tension to the printing sheet so that a wider portion thereof is maintained in a flat state.
FIG. 13 is a cross-sectional view of the printing apparatus of the ink-jet type, mainly showing a conveying mechanism for a printing sheet.
In FIG. 13, a printing head unit 7 mounted on a carriage 5 scans in the vertical direction relative to a paper surface and ejects ink during the scanning to carry out the printing. The printing sheet P is conveyed generally in the horizontal direction from a right side to a left side in the drawing beneath the carriage 5, for the printing area by the printing head unit. More specifically, on the upstream and downstream sides of a printing sheet conveying path, a set of a conveying roller (hereinafter referred also to an LF roller) 36 and pinch roller 37 and another set of discharge roller 41 and a spur 42 are provided, respectively, as the above-mentioned two sets of the conveying mechanism. Among them, the pinch roller 37 is pivoted on a rotary shaft provided in a pinch roller holder 30 and is pressed onto the conveying roller 36 by that the pinch roller holder 30 is biased by a pinch roller spring 31. Similarly, a pressure is also applied between the discharge roller 41 and the spur 42 by a pressing mechanism not shown. Thus, the printing sheet P is nipped by the two sets of rollers, and the conveying roller 36 is made rotate by a motor not shown. With this, the discharge roller 41 connected to the conveying roller 36 via a predetermined gear trains is made rotate, whereby the printing sheet P is intermittently conveyed at a predetermined distance at every scanning motion of the printing head.
In this regard, according to the above-mentioned conveying mechanism, when a rear end portion of the printing sheet exits from a nipped portion between the conveying roller and the pinch roller, the printing sheet may be discharged in the conveying direction by the biasing force of the pinch roller. In such a case, the conveying roller and the discharge roller further rotate by a backlash of the gear trains driving such rollers, whereby the printing sheet P may be conveyed more than a predetermined intended value. As a result, a position of the printing head relative to the printing sheet P is shifted from a regular position, whereby a position of a dot formed on the printing sheet P by the ink ejected from the printing head (an image position) deviates to degrade the quality of the printed image.
FIGS. 14A and 14B illustrate the positional relationship between the conveying roller 36 and the pinch roller 37. As shown in FIG. 14B, the conveying roller 36 has a length corresponding to a width of the printing sheet P to be conveyed, while a plurality of the pinch rollers 37, each having a shorter length, are disposed corresponding to the conveying roller 36. In this structure, when the rear end of the printing sheet P exits from the nipped portion between the conveying roller 36 and the pinch roller 37, the pinch roller 37 shift toward the conveying roller side at a distance corresponding to a thickness of the printing sheet P nipped thereby until this instant. According to this shift, the above-mentioned biasing force of the pinch roller generates, and the excessive length of the printing sheet P more than the predetermined value is conveyed as described above.
Particularly, if the printing sheet has relatively large thickness as glossy paper, the above-mentioned biasing force of the pinch roller becomes large to increase the excessive amount of the printing sheet to be conveyed.
As a countermeasure to the above-mentioned conveyance error, a system may be considered in which a brake is provided against the rotation of the conveying roller to restrict the excessive conveyance of the printing sheet upon the exiting thereof from the nipped portion. In such a case, however, there is a problem in that a loading torque for driving the conveying roller becomes large, whereby a grade of a motor for driving it must be grade-up or the conveying speed becomes insufficient.
On the other hand, Japanese Patent Laid-Open No. 2002-254736 discloses a printing method that reduces the displacement of the image position caused by the excessive conveyance of the sheet upon exiting from the nip of the roller sets as described above, wherein the displacement of the printed position due to the above-mentioned excessive conveyance is absorbed while allowing the increase in a conveyance amount upon exiting from the nip but shifting the nozzles used for printing corresponding to increased conveyance amount.
In the method disclosed in Japanese Patent Laid-Open No. 2002-254736, however, it is necessary to provide correction nozzles for the purpose of shifting, which is not used for the printing in a normal area, whereby the driving control of the printing head is relatively complicated.
Also, it may be considered that the excessive conveyance of the printing sheet is prevented by separating the conveying roller from the pinch roller before the rear end of the printing sheet exiting from the nipped portion between both the rollers to release the printing sheet from the nipping of the rollers. In this case, however, since the printing sheet is nipped solely by one set of discharge roller and the like, the conveyance accuracy thereof is degraded to deteriorate the quality of the printed result.
In this regard, there are many cases wherein a slight variation of the ejecting direction exists in the respective printing head of the ink-jet method. That is, due to the deviation of the ejecting direction from the regular direction, the landing positions of the ink droplets on the printing sheet may also similarly vary. Such variation of the landing positions, however, is negligible on the printing quality since the above-mentioned deviation of the ejecting direction itself is relatively small and a distance between the printing head and the printing sheet (hereinafter also referred to as “a head-medium distance”) is determined to be relatively small. Also, when the variation of the landing positions is significant, a so-called multi-pass printing may be adopted as a countermeasure thereto. When the head-medium distance is widened, however, the above-mentioned deviation of the ejecting direction enlarges the displacement of the landing positions.